Keep measurements on the level

This article looks at six technologies — mechanical floats and displacers, differential pressure, capacitance, ultrasonic, radar, and guided wave radar — that are used most often for automated control, and provides practical guidance for choosing among them.

By Jerry Boisvert, Siemens Energy & Automation

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Achieving effective automated control at a plant may well depend upon accurately knowing the level at any given time in a vessel, silo or other container. Unfortunately, with more than 20 technologies for continuous level measurement available, it can be confusing to sort out the most appropriate option for a given application. Variables such as temperature, pressure, vapor density, dielectric properties and many more come into play in making the right selection.

So, in this article, we will look at six technologies — mechanical floats and displacers, differential pressure, capacitance, ultrasonic, radar, and guided wave radar — that are used most often, and provide practical guidance for choosing among them.

The technologies

Let’s start with how the various methods work and their basic advantages and limitations.

Mechanical floats and displacers. This technology utilizes the Archimedes principle — the buoyant force on an immersed object equals the weight of fluid displaced. When liquid level rises, the weight of the displacer decreases linearly with the level (Figure 1). Outputs can be pneumatic or analog 4-to-20-mA DC.

Figure 1. Such devices are simple and inexpensive but require liquid of constant density.

Figure 1. Such devices are simple and inexpensive but require liquid of constant density.

This type of technology is simple and easy to install, and works well on liquids that are clean. It adapts well to a wide variety of fluid densities.

However, the fluid density must not vary, because the float or displacer is sized to the density of the liquid. If the density changes, then the weight of the displaced material changes, necessitating re-calibration. Another limiting factor for mechanical technology is build-up of material on the displacer or floats. This build-up also will cause a change in weight displacement and require re-calibration or cleaning of the displacer.

Differential pressure. The high pressure side of a differential pressure instrument is connected to the bottom of a tank and the low pressure side to the vapor space at the top of the vessel (Figure 2). The measured differential is the pressure head of the liquid in the tank. This is a true liquid level indication, if density doesn’t change.

Figure 2. The pressure difference between taps indicates the liquid head in the vessel.

Figure 2. The pressure difference between taps indicates the liquid head in the vessel.

A differential pressure transmitter suits liquids that are relatively clean and free of suspended solids. It is readily installed on a vessel and can be removed with the use of block valves.

However, it is of vital importance that liquid contents maintain a constant density. Density changes, such as those caused by variations in temperature or chemical make-up of the liquid, induce errors and mandate re-calibration.

Capacitance. Here, a contacting metal probe, usually lined with a sleeve of inert chemical-resistant material like PTFE, is attached to electronics with either a fixed or a variable frequency oscillator circuit. Application of a constant voltage to the rod or sometimes a flexible cable results in a frequency that infers level (Figure 3).

Figure 3. This technology can handle a wide range of applications but is

Figure 3. This technology can handle a wide range of applications but is intrusive.

This technology can handle a broad range of applications from cryogenics to extremely high temperatures and pressures, requires only one opening, usually at the top of the container, and has no moving parts to plug or wear out over time. For a conductive or water-based fluid, the application is relatively easy, with only an initial calibration needed unless a conductive coating clings to the probe.
The geometry of the container and other factors can complicate getting proper results. Moreover, capacitance is an intrusive measurement; so chemical compatibility with the vessel contents must be taken into account, as well as the potential for build-up problems. Additionally, for non-conductive or insulating fluids like hydrocarbons, the liquid chemistry must remain constant or homogeneous. Changes in material properties due to temperature or chemical make-up cause the dielectric property of the material to change, incurring errors and necessitating re-calibration.

Ultrasonic. This technology works by providing voltage to a piezoelectric crystal that sends sound waves through the air to the liquid or solid surface where reflection back to the transducer takes place (Figure 4). The distance, which is proportional to the travel time, relates to level. Ultrasonic technology, because of advancements in electronics and echo-processing software, has become one of the best options for continuous level measurements.

Figure 4. Chosen for environments containing heavy dust or high humidity.

Figure 4. Chosen for environments containing heavy dust or high humidity.

The technology measures from the top down; so there is virtually no contact with the contents of the container. Non-contacting technologies incur fewer problems with build-up, direct chemical attack and thus corrosion, and give a more consistent measurement. Ideally liquid should be homogeneous, with the temperature throughout the container fairly constant. In many cases, ultrasonic can measure solids, thanks to the ability of good signal-processing software and the use of flexible aimers for materials producing angles. Configuration, which was an issue years ago, now has become simple.

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